A controversial proposal to ask new mothers who have been raped for verification if they wish to claim tax credits for more than two children has become law without any debate or vote in parliament.

The so-called rape clause, which will be applied as part of new restrictions on tax credit entitlement, was added to legislation late on Wednesday as an amendment to an existing act. The plans will come into effect within a month.

Alison Thewliss, the SNP MP who first highlighted the issue of the clause, led a furious response, saying it was an “underhand parliamentary tactic” to introduce the measure without proper scrutiny.

She also described as “frankly astonishing” the fact that the Department for Work and Pensions (DWP ) had introduced the scheme apparently without having trained any of the people who will judge whether someone claiming the exemption has been raped.

The regulations, announced in the 2015 budget by the then chancellor, George Osborne, were put into law through a statutory instrument, a form of legislation that allows laws to be amended without parliament’s approval.

The policy coming into force on 6 April restricts tax credit entitlement for new claimants to a maximum of two children, with exceptions for multiple births and for women who could show that their third or subsequent child was conceived as a result of rape.

Those seeking to claim the exemption for rape must be assessed by what the government has described in a consultation as a “professional third party”, which could include health workers, police, social workers or rape charities.

With the regulation about to come into force, however, there is no indication of how a woman who has been raped could begin the process of claiming the tax credit exemption, or whether any of the third parties have been trained.

Thewliss said she had tabled a motion of prayer for annulment, a somewhat arcane version of the early day motion, in which a group of MPs sign a paper calling for a statutory instrument to be scrapped.

Such motions can be used to register displeasure with a government plan, but they rarely overthrow statutory instruments. According to Thewliss, the last time one was annulled in this way was 1979.

“Using such an underhand parliamentary tactic to railroad the rape clause into law is just the final insult this government could possibly dish out,” she said. “Not only did ministers sneak out their shameful consultation response as the eyes of the world were watching Donald Trump’s inauguration, but they’re now trying to put the rape clause on the statute books without a vote or debate, let alone any detailed scrutiny by MPs.

“The government must accept this policy is unworkable as well as immoral. With just days until nurses, doctors and social workers are expected to verify whether women had their third child as the result of rape, it’s clear there’s been no sexual violence training for those working with such vulnerable women.

“This is frankly astonishing, especially when you consider that the government is trying to railroad this through using medieval parliamentary procedures.”

The plan has caused worry among other MPs and women’s charities, with a number calling for it to be dropped. The DWP announced in January that it was to go ahead after the consultation.

The department has described the changes to tax credits as “a key part of controlling public spending”, and has pledged to implement the exemptions in “the most effective, compassionate way”.

A government spokesman said: “It’s absolutely right that we have the right exemptions in place and we have thought carefully about how we will work with charities and health and social care professionals to support victims of rape. We will be publishing guidance shortly.”